An example of this type of diaphragm is described in German patent application P 41 43 371.8-15. This type of diaphragm has a surface which is contacted by the product being conveyed. As a result, it is possible for the surface to be made flat, even and uniform to avoid any internal indentations in which particles may be deposited. In addition, there is only one sealing surface facing the central housing. The diaphragm can thereby be sealed between the central housing and the housing cover while at the same time being fixed in position as is necessary particularly when diaphragms of made solid PTFE-PFM, without a metal core or reinforcing plates, are used.
The diaphragm is constantly pressed against the central housing by the clamping ring. Two of the three surfaces act radially and are independent of the tension in the tension rod which holds the housing cover to the central housing, and one surface acts axially for the greatest possible security. When the housing cover is removed the diaphragm remains seated firmly and sealingly in the central housing thereby protecting the product being conveyed.
In this way, the functions of securing in position and sealing are separated from one another, and the parts performing these functions can each be of optimal form. Sealing of the diaphragm to the housing is effected by the clamping ring placed in the groove in the grooved bead of the diaphragm, which forces the exterior of the two large concentric annular surfaces of the grooved bead of the diaphragm into sealing contact with the interior walls of a corresponding groove in the housing. Thus, these annular surfaces are always kept under pressure by the clamping ring arranged between them, independently of the axial prestressing of the pump, i.e., the clamping force applied by the tension rod holding the housing parts together. Even an axial gap as much as a millimeter wide between the parts of the housing does not lead to loss of tightness or to the diaphragm being pulled out. As a result of this arrangement, servicing and monitoring of the axial prestressing is obviated.
The diaphragm according to the patent application referred to thus exhibits a series of advantages which make it very suitable for use in a double diaphragm pump for conveying high-purity products. However, since the components of the housing which, for reasons of corrosion resistance, consist of fluorothermopolymers of the PTFE family tend to deform under load, the constant prestressing by the diaphragm clamping ring, which is preferably formed from an elastomer of high Shore hardness, can gradually decrease during use of the pump. In addition, the continual stress applied by the clamping ring can also lead to loss of tightness when the pump is at a standstill for prolonged periods because the clamping ring applies force even when the pump is not in use. As a result, the surrounding parts yield and the originally predetermined prestress is thereby reduced. Moreover the high level of prestressing of the diaphragm clamping ring that is required makes assembly of the pump difficult.